There has been conventionally known a fuel vapor treatment apparatus that causes a canister to temporarily adsorb fuel vapor produced in a fuel tank and introduces the fuel vapor desorbed from the canister as required into an intake passage of an internal combustion engine to purge the fuel vapor. As one kind of fuel vapor treatment apparatus like this is proposed a fuel vapor treatment apparatus that measures the concentration of fuel vapor in an air-fuel mixture introduced into an intake passage before the fuel vapor is purged and controls an air-fuel ratio in the purged air-fuel mixture with accuracy. In fuel vapor treatment apparatuses disclosed in JP-5-18326A and JP-6-101534A, the flow rate or the density of the air-fuel mixture in a passage for introducing an air-fuel mixture into an intake passage is detected and the flow rate or the density of air in a passage open to the atmosphere is detected and the concentration of fuel vapor is computed from the ratio of these measurement results.
In these fuel vapor treatment apparatuses, negative pressure in the intake passage is applied to respective passages to pass the air-fuel mixture or air through the respective passages and at the same time the flow rate or the density of the air-fuel mixture or air is detected. Therefore, when the negative pressure in the intake passage pulses, the flow rate or the density fluctuates and hence the concentration of fuel vapor computed on the basis of the detection results of such flow rate or density deteriorates in accuracy. Moreover, when the negative pressure in the intake passage is small, the flow rate of the air-fuel mixture or air in each passage decreases and hence detection itself of the flow rate or the density of the air-fuel mixture or air cannot be preformed.
Therefore, the present inventors have earnestly conducted research on a fuel vapor treatment apparatus that reduces pressure in a detection passage having a restrictor by a pump and passes air and an air-fuel mixture through the detection passage and at the same time monitors a change in pressure difference between both ends of the restrictor and computes the concentration of fuel vapor on the basis of the monitoring results. In such a fuel vapor treatment apparatus, because pressure in the detection passage is reduced by the pump, a pressure difference to be detected is made stable except when detection conditions are changed and the flow rate of air or air-fuel mixture can be sufficiently secured in the detection passage. However, the results of research further conducted by the present inventors revealed that it was difficult in the construction of reducing pressure in a detection passage simply by a pump to make a detection gain G (refer to FIG. 45), which is expressed by a difference value between a pressure difference ΔPGas when an air-fuel mixture having a vapor concentration of 100% (hereinafter referred to as “100% concentration air-fuel mixture”) passed through the restrictor and a pressure difference ΔPAir when air passed through the restrictor, sufficiently large with respect to the resolution of pressure of a sensor. This results from the following fact: the flow rate of gas at the restrictor is proportional to the square root of the density of the gas and because a difference in density between air and air-fuel mixture is comparatively small, a difference value between pressure differences ΔPGas and ΔPAir, which are expressed by intersecting points of pressure difference (ΔP)−flow rate (Q) characteristic curves CGas of 100% concentration air-fuel mixture and CAir of air at the restrictor and a pressure (P)−flow rate (Q) characteristic curve CPump of a pump, that is, a detection gain G also becomes small. When a sufficiently large detection gain G cannot be secured like this, the relative detection accuracy of the pressure difference ΔPGas to the pressure difference ΔPAir and by extension the computation accuracy of the concentration of fuel vapor are reduced, which is not preferable.
For the above-mentioned reason, the object of the present invention is to provide a fuel vapor treatment apparatus capable of adjusting the flow rate of purge of fuel vapor with accuracy on the basis of state of the fuel vapor.